Journal of Young Investigators RESEARCH REVIEW The Structure and Physiology of the Nuclear Pore Complex and its Role in Gene Expression and Human Disease Kevin Doello1* The Nuclear Pore Complex (NPC) is a protein channel that communicates and transports molecules between the cytoplasm and the nucleoplasm. It has a complex structure composed of many structural proteins, mainly nucleoporins and transporter proteins, and its biosynthesis is an extremely regulated and cell cycle phase dependent process. Moreover, in the present review, it will be shown that this structure could play a decisive role in gene expression and in the pathogenesis of several diseases. On the one hand, NPC affects gene expression by regulating epigenetic enzymes and affecting the nucleocytoplasmic balance of transcription factors and small nuclear RNAs. On the other hand, diverse studies show that NPC is involved in some diseases and pathological processes like aging, neurodegenerative diseases and extrapyramidal syndromes, cancer, cardiovascular diseases, infectious diseases and genetic syndromes. In conclusion, NPC might be an important element in the control of gene transcription and cell phenotype, and perhaps, it will become a future pharmacologic target for several diseases in which it is involved in. INTRODUCTION The Nuclear Pore Complex (NPC) is a cellular component that is cytoplasmic filaments, and nuclear ring. They are all made by located within the nuclear envelope. It is a protein channel that approximately 30 different proteins called nucleoporins allows macromolecules to cross from the cytoplasm to the (D’Angelo and Hetzer 2008). nucleoplasm and vice versa. This establishes an important Nuclear basket is composed of a ring of nucleoporins physiological process known as nucleocytoplasmic transport (Nup153 and Nup50) which bind filaments of Tpr protein, (Jamali et al. 2011). NPCs are not only involved in transport but making up a structure with a basket shape (Brohawn et al. 2009; also have an important relationship with epigenetic enzymes and Hoelz et al. 2011). Cytoplasmic filaments are made up by the transcription factors, which are both proteins that participate in nucleoporins Nup88, Nup214 and Nup358 (Brohawn et al. 2009; gene expression (Van de Vosse et al. 2011). Moreover, it has Hoelz et al. 2011). Finally, the nuclear ring has four parts, been demonstrated that the NPC is linked to many diseases such namely, the membrane ring, the spoke ring, the inner ring or as aging, neurodegenerative diseases and extrapyramidal nuclear ring, and the outer ring or cytoplasmic ring. The syndromes, cancer, cardiovascular diseases (arrhythmias and membrane ring anchors the NPC to the nuclear envelope and sudden death), infectious diseases and genetic syndromes. For comprises nucleoporins gp210, Ndc1 and Pom121. The spoke example, in cancer or genetic syndromes, mutations exist in the ring is located centrally in respect to the membrane ring and is proteins that form the NPC (Capelson and Hetzer 2009). The built by nucleoporins Nup53, Nup54, Nup58, Nup62 Nup93, objectives of this review are to revise the structure and Nup98, Nup155, Nup188 and Nup205 (Brohawn et al. 2009; physiology of the NPC and investigate its role in gene expression Hoelz et al. 2011). Finally, the outer and the inner rings are and diseases in order to clearly understand the possible role of composed of nucleoporins Nup37, Nup43, Nup85, Nup96, the NPC as a gene expression regulator and a future therapeutic Nup107, Nup133, Nup160, Sec13 and Seh1 (Alber et al. 2007; target. Beck et al. 2007; Brohawn et al. 2009; Cronshaw et al. 2002; Hoelz et al. 2011; Lim et al. 2008; Stoffler et al. 2003). Another Structure of the nuclear pore complex protein called Aladin or Adracalin (Cronshaw and Matunis 2003) The NPC is an octagonal protein channel (Fabergé 1973) that is is located at the NPC as well and binds to a nucleoporin named between 40 and 90 nm in diameter (Webster et al. 2009) and 200 Ndc1 (Kind et al. 2009; Yamazumi et al. 2009) (Figure 1). nm in depth (Gall 1967). There are approximately 2000 NPCs in The nucleoporins that form the NPC can be classified a cell. Nevertheless, this number can vary depending on the cell into three groups: transmembrane nucleoporins (approximately type studied, the phase of the cell cycle and external aggressions 10%); structural nucleoporins (50%); and FG-nucleoporins (40% (Maeshima et al. 2011; Maul et al. 1980). of the total population) (Rout and Wente 1994). There are three structures within a NPC: nuclear basket, The deepest nucleoporins in the NPC project some of their peptide fragments into the pore hole. These peptide 1 School of Medicine, University of Granada, 18071 Granada, sequences are natively unfolded and hence are polypeptides Spain without a secondary structure folding. They make up a peptide web within the pore complex hole. In this web there are *To whom Correspondence should be nucleoporins containing GLFG (glycine-leucine-phenylalanine- addressed: [email protected] glycine repeats), FXFG (phenylalanine-x-phenylalanine-glycine repeats) and FG (the most common type) (phenylalanine-glycine 87 JYI | June 2013 | Vol. 25 Issue 6 2013 Journal of Young Investigators Journal of Young Investigators RESEARCH REVIEW repeats). These FGs are extremely important for the NPC’s Function of the nuclear pore complex involvement in the nucleocytoplasmic transport of NPC is responsible for the exchange of macromolecules and macromolecules, as FGs function as the anchoring points for the other substances between the nucleus and the cytoplasm cargo, indicate the direction of transport and adapt to appropriate (nucleocytoplasmic transport). There are two forms of transport. pore size (Denning et al. 2003; Terry and Wente 2009; Yang 1) simple diffusion, which occurs in molecules smaller than 40 2011). kDa (e.g. water, ions and metabolites) (Keminer and Peters 1999) The NPC is tightly bound to the nuclear membrane and and 2) nuclear translocation, for macromolecules like mRNA, the nuclear lamina by protein Pom121 and another NPC proteins, snRNA, tRNA, microRNA, transcription factors, histones, respectively (Hawryluk – Gara et al. 2005; Mitchell et al. 2010). enzymes and even viral proteins and nucleic acids (Ribbeck and The protein Pom121 comprises membrane ring in the NPC Görlich 2001). (Mitchell et al. 2010) and anchors NPC to nuclear membrane, Nuclear translocation can happen in two directions: while other NPC proteins are involved in the anchorage with nuclear import, in which molecules go into the nucleus, and nuclear export, a phenomenon where molecules go out to the cytoplasm. Both of them are extremely regulated processes and require transporter proteins (Jamali et al. 2011). Transport proteins involved in nucleocytoplasmic transport Transporter proteins can be classified into two groups: energetic molecules and anchoring molecules. Both of them are located in the NPC and are fundamental for nucleocytoplasmic transport (Marelli et al. 2001). Ran is a protein, from the family of Ras, which supplies the energy needed for the nucleocytoplasmic transport. It binds GTP or GDP, so it carries phosphate groups between the nucleoplasm and the cytoplasm (Sekiguchi et al. 2000). Anchoring transport proteins are called kariopherins. There are two kinds of kariopherins, namely exportins and importins (Gorlich and Laskey 1995; Marelli et al. 2001). Exportins, a protein group with seven elements, transport cargo from nucleus to cytoplasm (nuclear export) after recognizing them by a peptide sequence called nuclear export signal (NES) which is rich in leucine (Kutay and Güttinger 2005). On the other Figure 1. The nuclear pore complex (NPC). This image shows the main hand, importins transport molecules from the cytoplasm to the parts in which the NPC is divided and the specific proteins that compound nucleus (nuclear import) when a sequence known as nuclear them. Nuclear basket (in orange) is composed of Nup153 and Nup50 which bind filaments of Tpr protein. Cytoplasmic filaments (in purple) are localization signal (NLS) is located in the cargo (Kalderon et al. made up by the nucleoporins Nup88, Nup214 and Nup358. The nuclear 1984). ring has four parts, namely, the membrane ring (in brown), the spoke ring Importins are more complex than exportins because (in red and grey), the inner ring or nuclear ring and the outer ring or they are composed of two subunits called importin A and cytoplasmic ring (both in green). The membrane ring anchors the ring importin B that work together, but in some cases they can also complex to the nuclear envelope and comprises nucleoporins gp210, Ndc1 work separately. There are many forms of importin A and B, and and Pom121. The spoke ring comprises the nucleoporins Nup53, Nup54, consequently, lots of different combinations exist as well Nup58, Nup62 Nup93, Nup155, Nup188 and Nup205. The outer and the (Gorlich et al. 1995) inner rings are composed of nucleoporins Nup37, Nup43, Nup85, Nup96, Nup107, Nup133, Nup160, Sec13 and Seh1. Other important proteins are Nuclear Import also show in the picture. Nuclear import starts with a cytoplasmic protein that contains a NLS that is recognized by importin A, which then subsequently binds to Importin B. Although, this is the paradigm, many NLS nuclear lamina (A-lamina and B-lamina proteins) Lamina are recognized by importins that are not made up by two determines the location of NPC, because NPCs are more frequent subunits. The Cargo NLS – Importin A – Importin B complex in places of the nuclear envelope with B – lamina and less interacts with the elements of the NPC, more exactly with the FG common in the regions with A – lamina. Nup53, Nup153 and - Nups, binding to FG, GLFG or FXFG nucleoporins Pom121 bind to B – lamina, while Nup 88 binds to A – lamina temporarily. Not all FG - Nups have the same affinity to the (Hawryluk – Gara et al. 2005; Hawryluk – Gara et al. 2008; importin (especially importin B), existing FG - Nups called Lussi et al. 2011; Mitchell et al. 2010; Schwarz – Herion et al. clusters with more binding power (Bayliss et al.